When the current system is no longer useful, then it has to be replaced with new system. The system design involves designing of a new system or computer application that will meet the requirements identified during system analysis. System design is concerned with the computer oriented design of the system.For example- the detail of input transactions, the details of the printed reports, screen and other output, the file or database structure, contents of the records etc.
After analysis phase is complete, the system design phase begins. This phase is the most creative and challenging phase of SDLC. It is concerned with the design of new system. it involves designing of various things such as output design, input design, files design, processing and general program design etc. this state consists of logical design and physical design of the system. Logical design refers to the business aspect of the system and when the logical system is turned into technical specification it becomes physical design.
The types of system design are:
Logical Design: Theoretically designing of the system is called logical design. The system could be designed on the basis of the requirements. The logical requirement of the system is defined for the further designing of the proposed system. the main idea of this part is just the creation of the logic part of the program.
Physical Design: The conversion of logical design into designing tools and techniques is called physical design. It is more detail and complex jobs describing the solution of the problem. Most of the time it is designed on paper based and sometimes it is designed on computer system by using particular software. It uses algorithms, flowcharts, pseudocodes, decision table, decision tree. E-R Diagram, Data Flow Diagram, context diagram on paper based and MS-Visio is an example can be used as CASE (Computer Aided Software Engineering) tools to design physical design.
System Development
After the design stage is complete, the final program specification and the file design is handed over to the programmer. Then the programmers begin to develop the program by using a suitable HLL. In this stage, the programmer has to choose suitable programming language so that the program will be robust and it will be convenient to him. During this stage, the programmer has to detect all the syntax and logical errors and debug them. This stage is considered to be complete when the program can be compiled and executed.
System Testing
Once the program modules are ready, each of the program modules is tested independently as per the specifications of the users and debugged. When the modules are bug free, they are integrated as a single system and this system is tested entirely. If the entire system is bug free and can fulfill the requirements, then it is ready to implement.
There are two types of testing approaches:
Top-down approach: This approach is performed as testing main or top module at first and shifting towards lower modules is called top-down approach. This approach is verified from top to lower level until the existing lower modules.
Bottom-up approach: This approach is performed as testing lower-level modules at first and shifting towards up until the main module is tested is called bottom-up approach. It is the opposite of top-down approach.
There are two types of testing methods:
White Box Testing / Glass Box Testing:
The testing is performed with viewing all internal details are called white box testing. In white box testing, the internal source code of a program is tested such as operators, expression, control statement, variables, constant function etc. Programmer involves doing this job. The testing is done at source level of the software. This type of testing is time consuming and costly.
Black Box Testing / Functional Testing: The testing is performed without viewing all internal details are called black box testing. Instead of testing the coding part of the software, a program is tested to verify the desire outputs as per the given inputs. Different sample data is used to verify this kind of testing. Any user or person with testing tables can perform this testing. This type of testing is less time consuming and less costly.
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